Treatment of sewage and other waste water in its simplest form involves the mixing of waste water (food), air and micro-organisms to generate a biological process. This biological process breaks the "food" down into products more acceptable for disposal. The activated sludge ("AS") process developed in 1913 is now widely used, in various forms, for this purpose.
In the conventional activated sludge process, sewage, after primary screening and sedimentation, is passed into aeration tanks and then into a clarifier. A proportion of the biologically active sludge collected from the clarifier tank is recycled to the influent stream and constitutes about 25 to 200% of the inflow to the aeration tank. The balance of sludge is fed to an anaerobic sludge digestion unit.
In the aeration tanks, carbon-hydrogen compounds are oxidized as are reduced forms of sulphur, nitrogen and other elements. Nitrogen containing compounds are oxidized to nitrates ("nitrification"). In anoxic tanks nitrates are reduced to nitrogen gas ("denitrification").
In the sewerage treatment art, and as herein used, "anoxic" means in the absence of oxygen but presence of nitrate and "anaerobic" means in the absence of both oxygen and nitrate.
The activated sludge process has been developed as a continuous process in various forms, such as the Wuhrmann Process, the Ludzack-Ettinger and the Bardenpho processes. Those processes have in common that the influent is treated continuously and flows continuously in succession through a plurality of reactors in each of which the conditions are controlled for particular reaction conditions. Aerobic conditions are maintained in one or more reactors to promote nitrification and one, or more, reactors are maintained under anoxic conditions to promote denitrification. The outflow is settled and a proportion of the sludge is continuously recycled. These processes differ from one another primarily in the sludge recycling arrangements, the number of reaction chambers employed, and the selection of parameters used for primary control.
The continuous processes are capital intensive and useful for treating the wastewaters of large urban populations.
Along with development of the continuous process, the intermittent extended aeration process ("IEA") has been developed. In this process, a cyclic programme of aeration, settlement and decanting is performed in a single container. During the aeration phase, nitrification takes place. Anoxic conditions develop in the settlement phase during which denitrification occurs. The supernatant layer above the settled sludge is then decanted. Waste sludge is withdrawn and may be discharged to sludge lagoons or other sludge treatment process. During each step of the cycle, substantially the whole sludge content of the tank is subjected to the same conditions. The system is typically run with continuous inflow.
The IEA process in various forms has been operated with considerable success over 25 years or more and has the advantage that for communities of small to medium population, the plants are economical to construct, reliable, and relatively simple to control. The process generally achieves high removal of BOD, SS and nitrogen. It operates over a wide range of loads while tolerating rapid fluctuations in influent. Control is primarily achieved by regulation of oxygen input (aeration/non-aeration time) and sludge inventory.
In the 1970's interest developed in modifications to the continuous AS processes for the effective removal of phosphorus as well as nitrogen. The basic principle of biological phosphorus removal is the contact of "food" with micro-organisms under anaerobic conditions which cause the micro-organisms to convert soluble substrate to acetate while other selected microorganisms release phosphorus and absorb acetate. Under subsequent aeration, the stored acetate is used for energy production, for growth and to replenish the polyphosphate store. Thus, a greater amount of phosphorus is taken up than was released.
Nitrogen removal and phosphorus removal are to an extent competing processes because the presence of nitrates may inhibit phosphorus release and denitrification consumes soluble substrate needed for phosphorus removing bacteria growth.
Various modifications to the continuous process were proposed to achieve biological nitrogen and phosphorus removal, for example, the Phoredox and modified UCT processes. In consequence, the continuous processes became more complex to control and practical phosphorus removal required further increase in plant and operational complexity.
Since about 1982, various attempts have also been made to modify the IEA process to reduce concentration of phosphorus in addition to nitrogen. Typically, the unmodified IEA process achieves about 20% phosphorus removal. It has been shown that while aeration, settle, decant cycles were effective for nitrogen removal, removal of phosphorus with nitrogen can be achieved by introducing periodic anoxic mixing and sludge settling steps. Subsequent development has explored various strategies involving sequence including alternating cycles of anaerobic and aerobic conditions. However, to date, such IEA processes have not been operable with continuous feed, requiring that raw sewage be admitted only during the anaerobic phase. It has been found to be particularly difficult to control fluctuation in ammonia levels in the effluent and to maintain effluent ammonia concentrations of below 3 mg/L average. In general, the modified IEA processes hitherto proposed would be difficult to control and have not seemed practical.
An object of the present invention is to provide a process for waste water treatment which ameliorates at least some of the deficiencies of prior art. It is a further object to provide a process which retains the advantages of simplicity of control and comparatively low capital cost of the IEA treatment system while allowing for continuous inflow of waste water and resulting in effective phosphorus and nitrogen removal.